1. Field of the Invention
The invention relates to an apparatus and method to determine contact dynamics between conductive surfaces, and more particularly to determine the dynamics of contact closure in mechanical switches and analysis of the dynamics of other mechanical systems, in general.
2. Background Description
Switches are currently used in a wide variety of applications ranging from simple light switches to advanced microelectronic components such as RF switches using MEMs technologies. For example, mechanical switches such as toggle switches, rocker switches, and the like are used in a wide variety of digital, analog, and power systems. Switch design is a well established discipline, however, two conductive contacts can collide and bounce back and forth until a steady-state occurs, that is, switch contact bounce, although recognized is not often characterized in detail due to lack of convenient low cost measurement methodology. This same effect can occur in a wide array of different switch contacts (e.g., relays, circuit breakers), and in general to the dynamics of any reasonably conductive surface over time.
Manufacturers and users of switches need to be able to characterize a switch's performance over its lifetime. This is to enable optimization of the entire system. Some parameters of interest include:                (i) How long it takes a switch to reach steady-state;        (ii) The number of times the contacts bounce with each switch closure and opening;        (iii) The exact time the switches are in contact with each other at steady-state; and/or        (iv) The degree of separation versus time to determine, for example, if one line will be energized before another for multipole configurations.        
In digital systems each time the contacts bounce a separate electrical signal may be created resulting in several discrete events. These digital signals created by the bounce may result in malfunctions, false readings, and other effects to the system. To compensate for these effects, software routines are used to ignore extra signals during the period when the contacts are still bouncing. These “debouncing” software routines set a limit on how fast the switches can be activated, since if they are activated during the debounce period, they are ignored. Therefore, setting the duration of the debounce period is very important, and determining the statistical behavior of switch contact bounce assumes an important role.
Another switching application is in mechanical relay systems, that is, mechanical switches often used in high power systems. One such application is in high current circuit breakers used to protect wiring or the power distribution grid. In this type of system, the bounce of the contacts creates and/or prolongs the existence of an arc due to the high induced voltages that result when contact is broken. This plasma arc tends to pit and wear away the contact surfaces over time which, in turn, affects the lifetime of the contacts. Also, in circuit breakers, the arc represents circuit continuity that allows current to cross the switch even though it is open. Accordingly, a breaker designer needs to know the detailed dynamics of the breaker contacts over time in order to design a system that is capable of eliminating or extinguishing the arc using, for example, a high pressure inert gas jet. Also, in order to reduce the contact bounce in these systems, mechanical damping can be used. However, it is important to know the dynamics of the breaker contacts when using such a damping apparatus in order to evaluate its efficacy.
Techniques currently used to determine the dynamics of switch contacts during operation include, for example, high speed photography, optical interferometery, and accelerometers. However, each one of these techniques may be limited in its use to certain applications.
In high speed photography, high speed cameras may be utilized to photograph the contact of a switch over several cycles. These photographs can then be used to determine the dynamics of the contact during operation of the switch. However, while high speed photography equipment may be reliable, it tends to be very specialized and costly for this application. Moreover, the technique requires a high degree of setup. For example, when implementing high speed photography, optical access to the contacts is required. In a circuit breaker, a cut-away and installation of a transparent sidewall may have to be used to gain line of sight access. However, this may modify the electrical insulation properties of the switch itself.
At slower frame rates, for example, on the order of about 100 frames per second to about 1000 frames per second, strobe lights that have the desired repetition rate may be used to monitor the switch. In this type of configuration, line of sight access to the switch or the switching elements is still required. Also, this technique will not capture rapid mechanical movements, for example, on the microsecond time scale.
Another related art technique is optical interferometry. Optical interferometry uses interference patterns that result from reflections of a monochromatic light source such as a laser. This technique can be used to capture extremely precise measurements when there is a relatively slow motion, for example, measurements down to a fraction of a wavelength of the light may be captured. To use this type of system, a reflective surface is required in order to create an interference pattern. This technique is best utilized for extremely small distances that are outside the range of typical switching applications. Also, most conventional optical interferometers are designed for low speed movements rather than high speed movements that are present in switches. This technique also needs optical access to the moving contacts.
Another technique utilizes accelerometers to measure the acceleration of an object's mass. Typically, accelerometers include relatively massive reference mass in order to obtain good low frequency response. However, due to this relatively massive element, accelerometers are only able to sense the dynamics of larger (i.e., much more massive than the accelerometer) structures without corrupting the dynamics of the mechanical motion itself. That is, the weight of the accelerometer may affect the dynamics of a smaller structure, like a switch, resulting in an inaccurate portrayal of the switch dynamics. Thus, the accelerometer is typically used to measure vibrations of larger structures, for example, in an automobile, a building, an airplane or the like.